The electronic components used in the CPUs (Central Processing Units), etc. of servers, personal computers, etc. are needed to have high radiation efficiency of heat generated by semiconductor elements. To this end, such electronic components have the structure that a heat spreader of a material of high thermal conductivity, such as copper, is provided above the semiconductor element.
In this structure, because of fine concavities and convexities present in the surface of the heat source and heat spreader, a sufficient contact area cannot be provided even with both being in direct contact with each other. The contact interface becomes a large thermal resistance, and the heat radiation cannot be sufficiently made. For the purpose of decreasing the thermal contact resistance, the heat source and the heat spreader are connected with a thermal interface material (TIM) provided therebetween.
To this end, the thermal interface material is required to be a material of high thermal conductivity itself and furthermore have the characteristics which allow itself to contact over a large area with the fine concavities and convexities in the surfaces of the heat source and the heat spreader.
As the thermal interface material, heat radiation grease, phase change material (PCM), indium, etc. have been conventionally used. One of major characteristics that makes such materials usable as the heat radiation material is that they have fluidity at temperatures lower than the heat resistance temperatures of the electronic devices, which allows them to have large contact areas with the fine concavities and convexities.
However, the heat radiation grease and phase change material have low thermal conductivities as low as 1 W/m·K-5 W/m·K. Indium, which is a rare metal and additionally whose demand is much increased in the ITO related fields, has become expensive, and more inexpensive substitute materials are expected.
In such background, as the heat radiation material, the linear structure of carbon atoms represented by the carbon nanotube is noted. The carbon nanotube not only has very high thermal conductivity (1500 W/m·K-3000 W/m·K) in the longitudinal direction but also is good in flexibility and heat resistance, and has high potential as a heat radiation material.
As the heat conductive sheet using the carbon nanotube, a heat conductive sheet with carbon nanotubes dispersed in a resin, and a heat conductive sheet having oriented carbon nanotube bundles on a substrate buried with a resin or others are proposed.
The followings are examples of related: Japanese Laid-open Patent Publication No. 2005-150362; Japanese Laid-open Patent Publication No. 2006-100572; Japanese Laid-open Patent Publication No. 2006-147801; and Japanese Laid-open Patent Publication No. 2006-303240.
However, the conventional heat conductive sheet using carbon nanotubes cannot sufficiently utilize the high thermal conductivity of the carbon nanotubes.